PROJECT SUMMARY Pregnant women are up to five times more likely to be hospitalized with influenza than non-pregnant women, and young infants are more than four times as likely as older children to die from influenza. To protect pregnant women and their infants, the American College of Obstetricians and Gynecologists recommends that all women who are or will be pregnant during influenza season should receive an influenza vaccine. Based on current recommendations, influenza vaccination is currently part of standard prenatal care. Despite these recommendations and the demonstrated health benefits of prenatal vaccination, fewer than 40% of mothers are vaccinated during pregnancy, and vaccination rates are especially low during early pregnancy. Concerns for the safety of vaccination on pregnancy-related outcomes are commonly cited as contributing factors to this low rate of immunization. Few studies have evaluated the risk of spontaneous abortion (SAB) associated with influenza vaccination, a pregnancy outcome which may be a particular concern for women planning pregnancy or in the first trimester of their pregnancy. To date, what studies have been conducted have had major methodological flaws. Furthermore, no study has yet evaluated the impacts of paternal exposure to influenza vaccines or the health effects of pre-conception vaccination. Limited evidence in this area can present challenges to parents and their providers while making decisions on vaccination, thereby reducing public confidence in vaccination around the time of conception. This has been demonstrated by the lower confidence reported by physicians when recommending vaccination for women in their first trimester. A common limitation to existing vaccine studies is that recruitment begins during pregnancy, meaning that women in the earliest stages of pregnancy who are at highest risk of SAB and are less likely to be vaccinated are not included in these studies. This study aims to analyze data from a cohort of women and their partners recruited prior to conception in order to evaluate the association between the risk of spontaneous abortion and influenza vaccination during the weeks prior to and after conception. This study will consider both maternal and paternal vaccination as well as the number of vaccinations received in the previous year. To address this, the research team plans to analyze data from 11,150 female and 2,540 male participants of the Pregnancy Study Online (PRESTO) cohort, a North American prospective preconception cohort study. Influenza vaccination is currently considered the optimal strategy for protecting pregnant women and their infants against influenza. Using data from a large, cohort of women and their male partners, this application will address an important issue for prenatal care in the US and other countries. Study results will be useful to clinicians when evaluating whether to make vaccine recommendations and by families in their decision-making on whether to receive influenza vaccine around the time of conception.